Zooming in to a selection of vertices with the period key does not work as expected
It looks like you accidentally locked the view to an object or to the 3d cursor.

To unlock the view from object:


Press the N hotkey to open the properties panel on the right of the 3D view.
In the View section remove the object in the Lock to Object setting. If it is active, the Object name to which the view is locked will be listed in that field.
Click the X to the right of the object name to remove it.


To unlock the view from cursor:


Same as above, open the properties panel scroll down to the View section.
Uncheck the option "Lock to Cursor".


After unlocking the view, zooming to selection should work as expected.

In the image bellow you can see the correct settings for zooming to start working as expected.